A network interface device permits a subscriber to access a network. A passive optical network (PON) is an example of a network capable of delivering voice, video and other data among multiple network subscribers, using a common optical fiber link. Passive optical splitters and combiners enable multiple optical network terminals (ONTs) to share the optical fiber link. In a PON, each ONT terminates the optical fiber link for a residential or business subscriber, and is sometimes referred to as a subscriber premises node that delivers Fiber to the Premises (FTTP) services.
An ONT is connected to one or more subscriber devices, such as televisions, set-top boxes, telephones, computers, or network appliances, which ultimately receive the voice, video and data delivered via the PON. An ONT is an example of a network interface device. Other examples of a network interface device, in different network types, include cable modems and digital subscriber line (DSL) boxes.
Generally, a network also includes a network access interface. In a PON, the network access interface is sometimes referred to as an optical line terminator (OLT), having multiple, independent PON interface modules that serve multiple optical fiber links. A PON interface module provides an interface for transmission and reception of data packets over a particular optical fiber link that serves a group of ONTs. A PON is a downstream-multicast medium. Each packet transmitted on an optical fiber link can be received by every ONT served by that link. ONTs identify selected packets or frames on the fiber link based on addressing information included within the packets or frames.
Network equipment, such as network interface devices, in a cable or hybrid network may be connected to power provided by a central office (CO), which commonly utilizes battery and generator back-up power to maintain a continuous power supply. In contrast, an ONT in an all-fiber optic network is ordinarily powered locally at the subscriber premises. For this reason, an ONT often includes a battery to provide back up power during a power outage to maintain critical services, such as voice service. In particular, many FTTP service providers provide an uninterruptible power source (UPS) unit. A UPS unit provides AC-to-DC (alternating current to direct current) voltage conversion from line power within the subscriber premises, and includes a battery for backup power.
Battery resources are limited and depend on battery size and the rate of power consumption. Because battery performance and reliability are adversely affected by temperature extremes, a UPS unit is normally mounted inside the subscriber's premises. It is important that the battery-based power source be reliable in order to maintain critical services during a power outage. Because batteries are chemical systems that wear out over time, however, batteries must be replaced and are generally unreliable.
A UPS unit typically performs battery status monitoring functions on a continuous or periodic basis to ensure the health of the battery and the reliability of the FTTP network. A UPS unit may transmit battery status or alarm signals, such as AC Fail, Battery Low, Replace Battery, and Battery Missing, as discrete signals on separate wires to a network interface device, such an ONT. As a result, a UPS unit may have at least six wires: power, ground, and a wire dedicated to each status or alarm signal.